Post #G21-017: Warming a raised bed at night 3: passive solar helps.

Posted on April 20, 2021

This is a continuation of a series of posts on how to keep the surface of raised beds warm during cool spring nights.  So far I have shown:

  • What doesn’t work:  Floating row cover (Post #G21-012)
  • What does work:  Space blanket plus “passive solar” (Post #G21-014)
  • Why that works:  The math behind the space blanket (Post #G21-015)

Here, I answer two more questions.  Yes, you need the passive solar input (jugs of water than have warmed in the sun).  Or, at least, that clearly helps quite a bit.  And no, you won’t kill your plants if you forget to remove the space blanket the next day.

1)  Covering the raised bed with a space blanket is idiot-proof.  The question is, what happens if you forget to remove the space blanket in the morning?  If you do that with clear or black plastic, the noonday sun cooks the plants underneath.

Answer:  If you leave it on in full sunlight, the space blanket cools the bed underneath.  That makes the space blanket much more forgiving of “operator error”.  If you forget to attend to it in the morning, you won’t accidentally kill the plants in your raised bed.

2)  The solar-heated gallons of water matter.  I tried this last night with just the space blanket, no sun-warmed gallon jugs of water.  And, in accordance with the theory, I got some benefit, but not as much as I did with the passive solar input.

These next two graphs contrast the situation using a space blanket alone, or using a space blanket plus some gallon jugs of sun-warmed water (at about 75F at the start of the experiment).

With the space blanket alone, without the jugs of solar-warmed water, I got maybe five degrees of warming (top).  With the space blanket and jugs of lukewarm water, I got about ten degrees of warming (bottom).  That is, the gap between red (treatment) and blue (control) is visibly larger in the bottom graph than in the top graph.

This matches the underlying theory pretty well.  The raised bed itself stores some heat, but not a massive amount of heat.  And it’s fairly hard for that heat to flow out of the entire mass of dirt.  As a consequence, the heat input from just a few jugs of warm water (maybe one per six square feet of bed, in the test above) is significant, compared to the amount of heat that the bed itself can give off overnight (Post #G21-015).  Here, adding the jugs of warm water more-or-less doubled heating of the bed surface overnight.

In theory, I have two more experiments to do.  I should try this with (e.g.) a piece of common black plastic in place of the space blanket.  That will test whether or not it really is the radiant-barrier aspect of the space blanket that matters, or, by contrast, whether I’m seeing the warming merely because I’ve trapped some air next to the bed using a plastic sheet.  Finally, I should do that with and without the jugs of warm water.

As with this last experiment, I don’t expect those to be simple yes/no answers.  Instead, I expect to see that they don’t work as well as the combination of passive solar plus space blanket.  I’m pretty sure that’s the best combination in terms of total heating power.  I just need to verify that.